It will vary from area to area somewhat, but if you're going for the big "door busters" such as TV's, computers, etc. you'll usually need to line up several hours in advance (lines have formed starting at 6 pm at some locations), even then, you're not guaranteed to be at the front of the line.
If all you're looking for are smaller ticket items (DVD's, games, etc.), in my general experience you don't need to be there when the doors open. Also, there's the possibility that those deals will be on their website; so if you don't plan on being there when the doors open it may be worthwhile to see what you can pick-up online first. You can beat a lot of crowds that way.
Also, for the past few years Wal-Mart has priced matched deals. One note on this - if you want to price match another door buster, you need to be getting it priced checked before noon. In the past Wal-Mart said that they will honor only as long as the other store's are still having the deal.
One thing that waiting in line, online listings (depending on the store), and price-checking have in common: YMMV.